Nuclear medicine is a field of medicine concerned with the use of radiation for diagnostic purposes. Single Photon Emission Computed Tomography (referred to in this specification as “SPECT”), a branch of nuclear medicine, involves directly measuring gamma rays emitted by radionuclides administered to a patient to produce slice-like images of the patient. “Tomography” refers to the production of slice-like images, or tomograms. Computerized Tomography (CT) refers to the use of computer processing to derive the tomogram.
Typically, in SPECT procedures, radiopharmaceuticals (otherwise known as radioactive tracers or radiotracers) are administered to patients. Radiopharmaceuticals are generally compounds consisting of radionuclides (i.e. radiation-emitting atoms), combined with pharmaceuticals or other chemical compounds. In some cases, such as with Thallium-201, the same particle is simultaneously the radionuclide and pharmaceutical. Unlike positron emission tomography (PET), which uses small-nucleus radionuclides with half-lives ranging from just over a minute to under 2 hours, SPECT involves the use of radionuclides whose half-life is several hours to days long, long enough to clinically localize or become fixed in specific organs or cellular receptors. In these circumstances, it is possible to acquire important diagnostic information by obtaining images created from the radiation emitted by the radiopharmaceutical. In SPECT, cameras which receive and detect gamma rays emitted by the radiotracer are used in the imaging.
The end result of a SPECT procedure is a stack of tomograms that can be combined into a 3D image. To create the tomograms, the camera acquires multiple planar or projection images from different angles, This is typically done by one or more camera heads that rotate around the patient to acquire images from different angles, or less commonly by an immobile head that spans a substantial angular distance, and can thus simultaneously acquire multiple images from different angles. A computerized image reconstruction algorithm is then applied to the datasets from the planar images to produce a tomographic image dataset that can be combined to form a 3D image. It will be appreciated by those skilled in the art that the “3D” image is displayed on a flat computer monitor, and is thus not literally three-dimensional. Its three dimensional nature can only be appreciated if it is viewed, say, while spinning on a screen, or recreated as a sculpture.
SPECT cameras typically employ photon collimation, typically in the form of absorptive collimation, in which lead or occasionally tungsten is used to absorb (and thus eliminate from detection) most photons. In the case of parallel-hole collimators, the dominant form, all the photons that do reach the photon detector are collinear (i.e. have substantially parallel trajectories). Typical for parallel hole collimators the photons passed by the collimator have trajectories orthogonal to the planar photon detector. Other types of collimation may be used but all types depend on eliminating between 99.9% and 99.99% of incoming photons.
SPECT cameras are configured to detect photons, and to measure their energy and the location of their incidence on the photon detectors. Based on this information, and the fact that the photons are collimated, the precise trajectory of each incoming photon is known, assigning it a particular location on the planar image being created by the camera. This information is used by the computer in constructing the tomograms.
Knowing the appropriate location on the planar image to which each photon should be assigned is important because the information carried by SPECT images is conveyed, at least in part, via the relative intensity of activity in each part of the image. A portion of the image representing the source of a relatively large number of photons will be relatively intense and will generally be displayed as more intensely bright or dark, depending on the color scheme chosen for display. A portion of the image representing the source of a relatively small number of photons will be less intense.
For example, for Myocardial Perfusion Imaging (MPI), a radiotracer that emits gamma rays is injected into the patient. A feature of such radiotracers is that they are carried in the bloodstream into the myocardium (heart muscle), and become fixed in the myocardium. As a result, the distribution of radiotracer is indicative of relative blood flow through the myocardium. If a particular coronary artery is partially occluded, while the others are not, there will be relatively less radiotracer fixed in the portion of the myocardium supplied by that particular coronary artery. This would in turn be reflected in the SPECT image, where portions of the myocardium receiving greater blood flow would appear relatively intense, and those receiving less blood flow would be less intense.
However, there is a downside to the use of traditional absorptive parallel-hole collimation: between about 9990 and 9999 out of every 10,000 incident photons reaching the collimator are blocked out of the SPECT image. Only between about 1 and 10 out of every 10,000 photons are passed by the collimator to the detector to be included in the image. Other types of collimators (e.g. pinhole collimators) may block an even higher proportion of photons, with only a tiny percentage reaching the photon detector.